The invention relates to an x-ray camera, a use of the x-ray camera and a method for recording x-ray images with the features defined in the preambles of respective independent claims.
Sine the discovery of x-rays by Wilhelm Conrad Röntgen, different physical effects and methods have been used for detecting this very short wavelength radiation. This goes from detecting x-ray radiation with photographic films via ionization detectors and semiconductor detectors to modern x-ray cameras using luminophores and CCD arrays. This development was in particular advanced by medical diagnostic applications. A similar trend has been observed over the past decades in x-ray analytics, where other physical and technical boundary conditions had to be considered with respect to the x-ray photon energies, the energy resolution and the intensity. During the past 20 years, a trend for transitioning from compact single detector systems to spatially resolved detectors has occurred in both x-ray fluorescence spectroscopy and x-ray diffractometry. Initially, line cameras were preferably used, whereas during the past 10 years also area detectors have increasingly been employed. These detection systems are limited to detecting spatial information, i.e., individual pixels, which can form the basis for image representation, as well as detecting the intensity of the radiation for a single point (pixel).
The object of this patent is the physical and technical description of a novel x-ray color camera, which makes possible simultaneous spatial, energy and time resolution by using compact x-ray optics and a two-dimensional pixilated semiconductor sensor.
A two-dimensional image pickup element which is sensitive to x-rays and is structured across an area was for a long time only photo-chemically sensitized coated films and more recently also semiconductor chips, like an XMM-Newton or Chandra in x-ray telescopes orbiting the earth.
The radiation in non-image-forming radiation measurement devices (scintillation sensor, whole body counter or counting tube) is frequently collimated by simple conical tubes or lamellae. In a gamma camera, the collimator quasi resembles a perforated board, wherein the individual “holes” are separated by the so-called septa. These septa ensure that only the perpendicular rays can pass through the collimator, whereas photons incident at an angle are absorbed. The length of the septa which corresponds to the overall height of the collimator, and the width of the septa determine suitability for different photon energies (low-energy, medium-energy, high-energy and ultrahigh-energy). The sensitivity (measurement yield) depends on the ratio of hole width to the septa width.
The channel ratio (septa length to hole width) determines the permissible entrance angle and hence the spatial resolution, which is therefore limited by the septa thickness (required for the respective photon energy). Collimators therefore generally always require a compromise between, on the one hand, the sensitivity and, on the other hand, the resolution. The resolution increases with a smaller width and greater length of the bores which, however, reduces the sensitivity, and vice versa.
The parallel-hole collimators produce a parallel projection. The measurement yield is almost independent of the distance to the object; however, the spatial resolution deteriorates significantly with increasing distance to the detector. Parallel-hole collimators are hexagonally folded and soldered either from lead or tungsten sheets, or are drilled into a solid block, or cast. The bores are preferably filled with a plastic material transparent for the radiation. Slant-hole collimators are parallel-hole collimators with inclined bores and enable, for example, measurements at the shoulders closer to the head of a patient. Diverging collimators produce an enlarged image. In fan-beam collimators, the holes only diverge in one spatial direction. This increases the counting yield, because a larger crystal region receives the radiation originating from a small object.
The known pinhole collimators operate according to the principle of the pinhole camera: a single “pupil” produces a reversed and inverted image, with the magnification of the image depending strongly on the object distance. Pinhole cameras are used to image particularly small radiating objects (wrist bones, test animals, potentially also thyroid), because they allow a strong magnification compared to the aforementioned collimators. More recently, multi-pinhole collimators are used which increases sensitivity without decreasing the spatial resolution.
Spatially sensitive detectors for x-ray radiation are known since some time. U.S. Pat. No. 3,772,521 “Radiation camera and delay line readout” describes such detector wherein a proportional counter is equipped with anode and cathode wire grids. An electrical signal is generated by the incident x-ray photons, which is localized depending on the position by the respective readout wire grid from the corresponding time delay of the signal transmission. An improved version of such detection system is described, for example, in the published patent application DE 37 35 296 A1 “Two-dimensional proportional counter for spatially sensitive measurement of ionizing radiation.”
In addition, luminescent foils and storage foils are used as position-resolving detectors. These systems and the corresponding required readout devices are described, for example, in the patents EP 0421632 A2 “Digital x-ray image processing apparatus and method” and EP 0667540 B1 “Readout device for photo-stimulated information carriers”. However, these detectors are not as powerful as CCD detectors, which have significantly better characteristics with respect to both the readout speed as the spatial resolution. Such detector systems, which are used particularly for diffractometric image recordings in crystallography, are in existence since the development in the late 80s of the 20th century. However, these detection systems only register intensity differences, i.e., they have only spatial resolution.
U.S. Pat. No. 5,491,738 describes an x-ray diffractometer and US 2007/0041492 A1 an x-ray diffractometer microscope, wherein both solutions each have an x-ray source as well as an x-ray detector. None of the solutions includes x-ray capillary optics or a two-dimensional pixilated x-ray detector.
A pn-CCD detector structure which can be used for the x-ray color camera according to the invention was described in the patent application DE 10 2005 025 641 A1.
Poly-capillary structures are to be used as optical systems, which are formed as parallel structures or conical structures. The following publications from the year 1995: V. A. Arkadiev, A. Bzhaumikhov: “X-ray focusing by polycapillary arrays”, Proceedings of SPIE Vol. 2515 (1995) 514-525.) and from the year 1998: A. A. Bzhaumikhov, N. Langhoff, J. Schmalz, R. Wedell, V. I. Beloglazov, N. F. Lebedev: “Polycapillary conic collimator for micro-XRF”, Proceedings of SPIE Vol. 3444 (1998) 430-435 address this subject. Unlike the aforedescribed x-ray collimators for hard x-rays made from lead or tungsten, the poly-capillary structures made from glass do not significantly absorb the low-energy x-ray radiation for which they are designed, but instead produce a reflection which essentially deflects the low-energy x-ray radiation.
EP 1953537 A1 describes a device for measuring and/or guiding x-ray radiation by using an x-ray optics, wherein the x-ray optics has capillaries. This x-ray optics is used for measuring and guiding the x-ray radiation, but not for generating images. The employed x-ray detector lacks any spatial resolution.
It is an object of the invention to provide a practical electronic x-ray camera capable of capturing in a simple manner images having spatial and spectral resolution, realizing different image modes and hence a simultaneous spectral recording of low-energy x-ray radiation at each image pixel.